furanózes

Furanose is a cyclic form of a monosaccharide. It is characterized by a five-membered ring structure containing four carbon atoms and one oxygen atom. The term "furanose" is derived from furan, a five-membered heterocyclic organic compound containing a ring of four carbon atoms and one oxygen atom. Furanose sugars are formed when the hydroxyl group on the carbon atom adjacent to the carbonyl group (the anomeric carbon) reacts with the carbonyl group itself through an intramolecular hemiacetal or hemiketal formation.

In aqueous solution, monosaccharides like glucose and fructose exist in equilibrium between their open-chain and cyclic forms. Furanose rings are one of the two common types of cyclic structures, the other being pyranose, which has a six-membered ring. The specific structure of a furanose is determined by the number of carbon atoms in the original open-chain monosaccharide. For example, a six-carbon sugar like glucose can form a furanose ring, though the pyranose form is generally more stable and prevalent. Fructose, a six-carbon sugar, more readily forms a furanose ring.

The formation of a furanose ring involves the creation of an anomeric carbon. The configuration of the hydroxyl group attached to this anomeric carbon can be either alpha ($\alpha$) or beta ($\beta$), leading to different anomers of the furanose sugar. These anomers are in equilibrium with each other and with the open-chain form. Furanose structures are important in the structure of certain carbohydrates, including some nucleotides and disaccharides.